The present invention relates to a process for the production of vinyl acetate.
Vinyl acetate may be produced by the acetoxylation of ethylene in the presence of a palladium-containing catalyst. In addition to vinyl acetate, carbon dioxide is produced as a by-product.
In commercial operation, vinyl acetate product and unreacted ethylene are recovered from the gases exiting the reaction vessel. The unreacted ethylene is recycled, together with fresh ethylene to the reaction vessel. The recycled ethylene contains some carbon dioxide and other by-products as well as some inert gases such as nitrogen and/or argon. The presence of these components limits the concentration of ethylene in the feed to the reaction vessel.
The factors which influence the selectivity of the process towards vinyl acetate are not fully understood. The general view taken by those skilled in the art is that once the concentration of ethylene in the reactant mixture exceeds a threshold value, the rate of formation of vinyl acetate becomes independent of the concentration of ethylene. The rationale behind this view is that even if ethylene is involved in the rate determining step, the rate of formation of vinyl acetate with respect to ethylene will tend to zero once an amount greater than the stoichiometric amount of ethylene is exceeded. In other words, as long as an excess of ethylene is present in the reactant mixture, the magnitude of this excess should not have a bearing on the amount of vinyl acetate produced.
This general view or technical prejudice is supported by experimental data in Davidson et al. (Front. Chem. React. Eng., 1984, (1) 300-313). The data show that in acetoxylation reactions carried out at atmospheric pressure, the amount of vinyl acetate produced remains substantially constant as the ethylene concentration is increased between 31.6 mol % to 47.4 mol %.
The technical prejudice also extends to acetoxylation reactions performed above atmospheric pressure. In Abel et al. (Chem. Eng. Technol. 17 (1994) 112-118), ethylene, acetic acid and oxygen are reacted together at a total pressure of 8 barg. The document recites that the rate of formation of vinyl acetate is independent of the concentration of ethylene in the reaction mixture, provided that the concentration of ethylene exiting the reactor is above 30 mol %. To ensure that the effluent concentration exceeds this value, ethylene feed concentrations of 57 mol % are employed.
R S Shetty and S B Chandalia in Metals and Minerals Review December 1970 35-40 propose that in an industrial process an ethylene concentration of more than about 65% may be chosen to keep outside the explosive limit. However in an experiment using a gas mixture containing a mole ratio of ethylene to oxygen of 69.3:30.7 the catalyst activity changed rapidly with time. Furthermore, the concentration of acetic acid and hence the ethylene concentration is not apparent. Neither is it apparent whether ethylene was recovered and recycled.
Nakamura et al in J. Catal. 17 (1970) 366-374 describes the effect of potassium acetate on the catalytic activity of a palladium catalyst with a feed gas of ethylene: oxygen: acetic acid of 80:10:10 (FIG. 4). However it is not apparent that ethylene was recycled nor is any beneficial effect of high ethylene concentration described.
Samanos et al in J Catal. (1971) 23 19-30 describe the rate of reaction forming vinyl acetate varying linearly with ethylene partial pressure whilst the rate at which carbon dioxide is formed remains constant. However, it is not apparent the ethylene is recovered and recycled in the experiment.
We have now found that when vinyl acetate is produced above atmospheric pressure, the selectivity towards vinyl acetate increases as the concentration of ethylene in the reactant mixture is increased above 60 mol %. This finding is contrary to the technical prejudice in the art.
Accordingly, the present invention provides a process for the production of vinyl acetate, said process comprising the steps of:
(a) introducing ethylene, acetic acid and an oxygen containing gas into a reactor,
(b) reacting said ethylene, acetic acid, and oxygen-containing gas in the presence of a catalyst material in said reactor to produce vinyl acetate at above atmospheric pressure,
(c) withdrawing from said reactor gases comprising unreacted ethylene, vinyl acetate, carbon dioxide by product and inert gases such as nitrogen and/or argon;
(d) recovering from the gases withdrawn from said reactor, unreacted ethylene optionally together with minor amounts of said carbon dioxide and inert gases, and
(e) introducing the recovered ethylene from step (d) and additional ethylene to said reactor in step (a) wherein the amount of ethylene in the combined feed to the reactor is at least 60 mol % .
Preferably also, the method further comprises the step of recovering the vinyl acetate produced.
The present invention provides a novel and cost effective route for the production of vinyl acetate at above atmospheric pressure, 0 Pa gauge (0 barg). The reaction may be carried out at a pressure of between 5xc3x97104 Pa gauge (0.5 barg) and 2xc3x97106 Pa gauge (20 barg), preferably between 6xc3x97105 Pa gauge (6 barg) and 1.4xc3x97106 Pa gauge (14 barg) and, most preferably, between 7xc3x97105 Pa gauge (7 barg) and 1.2xc3x97106 Pa gauge (12 barg). These pressure conditions are believe to affect the intersection between ethylene and the surface of the catalyst such that an increase in selectivity towards the production of vinyl acetate is observed when the amount of ethylene in the feed is increased above 60 mol%. This effect is surprising: the rate of vinyl acetate production with respect to ethylene is expected to be of zero order once an initial ethylene concentration of at least 57 mol % is employed as taught in Abel et al. (Chem. Eng. Technol. 17 (1994) 112-118).
Whilst we do not wish to be bound to theory, the increase in selectivity may be explained by a combination of two factors. In addition to increasing the amount of vinyl acetate produced, an increase in the amount of ethylene in the feed is also found to reduce the amount of carbon dioxide produced as a by-product. This in turn, can reduce the amount of carbon dioxide which has to be separated from the recovered unreacted ethylene and/or the amount of carbon dioxide recycled to the reactor with the recovered ethylene.
The catalyst of the present invention may be fixed-bed or fluid-bed catalyst. Preferably, a fluid-bed catalyst is employed in a fluid bed reactor.
The catalyst suitably comprises a Group VIII metal and a promoter. Preferably, the catalyst further comprises a co-promoter. These compounds are suitably accommodated on a support.
With regards to the Group VIII metal, the preferred metal is palladium. The metal may be present in a concentration of greater than 0.2% by weight, preferably greater than 0.5% by weight, especially about 1% by weight based upon total weight of catalyst. The metal concentration may be as high as 10% by weight. Suitable sources of palladium include palladium (II) chloride, sodium or potassium tetrachloropalladate (II) (Na2PdCI4 or K2PdCI4), palladium acetate, palladium (II) nitrate, H2PdCl4 or palladium (II) sulphate.
In addition to the Group VIII metal, the catalyst comprises a promoter. Suitable promoters include gold, copper and/or nickel. A preferred promoter is gold. Suitable sources of gold include gold chloride, tetrachloroauric acid (HAuCl4), NaAuCl4, KAuCl4, dimethyl gold acetate, barium acetoaurate or gold acetate. The preferred gold compound is HAuCl4. The promoter metal may be present in an amount of from 0.1 to 10% by weight in the finished catalyst.
The catalyst composition may comprise a co-promoter material. Suitable co-promoters include Group I, Group II, lanthanide or transition metals for example cadmium, barium, potassium, sodium, iron, manganese, nickel, antimony, and/or lanthanum, which are present in the finished catalyst as salts, e.g. an acetate salt. The preferred salts are potassium or sodium acetate. The co-promoter may be present in the catalyst composition in a concentration of up to 15%. Where the catalyst is a fixed bed catalyst, the co-promoter concentration is preferably between 3 and 11 wt %. Where the catalyst is a fluid bed catalyst, and the acetic acid is introduced into the reactor in the liquid form, the co-promoter may be present in a concentration of up to 6 wt %, preferably between 3.5 to 5.5 wt % especially 5 wt %. Where the catalyst is a fluid bed catalyst and the acetic acid is introduced into the reactor in vapour form, the co-promoter may be present in a concentration of up to 11 wt % of the total catalyst.
The activity of the catalyst may decrease with time. In particular, due to the volatile nature of the co-promoter, the level of co-promoter in the catalyst material decreases with time, thus resulting in loss of catalyst activity. In order to maintain a constant concentration of co-promoter in the catalyst, fresh co-promoter may be added to the catalyst during the reaction. This may suitably be carried out by adding the co-promoter material to the liquid acetic acid feed or to the liquid recycle acetic acid. Alternatively, the additional co-promoter may be introduced as a solution e.g. in water or in acid directly by spraying through a suitable injection means such as a nozzle.
The catalyst material is a supported catalyst. Suitable catalyst supports include porous silica, alumina, silica/alumina, silica/titania, titania, zirconia or carbon. Preferably the support is silica. Suitably, the support may have a pore volume from 0.2 to 3.5 mL per gram of support, a surface area of 5 to 800 m2 per gram of support and an apparent bulk density of 0.3 to 1.5 g.mL. The support may typically have a particle size distribution such that at least 60% of the catalyst particles have a particle diameter of below 2xc3x9710xe2x88x924m (200 microns). Preferably at least 50%, more preferably at least 80% and most preferably at least 90% of the catalyst particles have a particle diameter less than 1.05xc3x9710xe2x88x924m (105 microns). Preferably no more than 40% of the catalyst particles have a diameter of less than 4xc3x9710xe2x88x925m (40 microns).
The catalyst may be prepared by any suitable method. Suitably, the first stage of the catalyst preparation process involves impregnation of the support material with a solution containing the required Group VIII metal and the promoter metal in the form of soluble salts. Examples of such salts are soluble halide derivatives. The impregnating solution is preferably an aqueous solution and the volume of solution used is such that it corresponds to between 50 and 100% of the pore volume of the support, preferably 50 to 99% of the pore volume.
The impregnated support is dried at ambient or reduced pressure and from ambient temperature to 150xc2x0 C., preferably 60 to 130xc2x0 C. prior to metals reduction. To convert such materials into the metallic state, the impregnated support is treated with a reducing agent such as ethylene, hydrazine, formaldehyde or hydrogen. If hydrogen is used, it will usually be necessary to heat the catalyst to 100 to 850xc2x0 C. in order to effect complete reduction.
After the steps described above have been carried out, the reduced catalyst is washed with water and then dried. The dried carrier is then impregnated with the required amount of co-promoter and thereafter dried. Alternatively, the wet, reduced, washed material is impregnated with co-promoter then dried.
The method of catalyst preparation may be varied to optimise catalyst performance based on maximising vinyl acetate yield and selectivity.
The process of the present invention comprises reacting ethylene, liquid acetic acid and an oxygen-containing gas in the presence of the catalyst material. Ethylene may be used in substantially pure form or admixed with one or more of nitrogen, methane, ethane, carbon dioxide and water in the form of steam or one or more of hydrogen, C3/C4 alkenes or alkanes.
The amount of ethylene in the combined feed to the reactor is at least 60 mol %. The upper limit to the ethylene concentration in the combined feeds is determined by a number of factors including (a) the need to have oxygen and acetic acid reactants in the feed and (b) the loss of ethylene in gas purge from the reactor to remove carbon dioxide and other by-products, and inert gases such as nitrogen and/or argon. Suitably, the amount of ethylene in the combined feed is up to 85 mol %. Preferably, the amount of ethylene in the combined feed is in the range at least 60 mol % to 80 mol %.
Operating with higher ethylene concentrations can lead to an increased loss of ethylene in the gases vented from the reaction system to remove carbon dioxide and inerts. However, if the amount of inerts in the system can be reduced, the amount of purge gas which has to be vented can be reduced and hence the loss of ethylene can also be reduced. The need to vent purge gas from the reactor to remove inert gases such as nitrogen and/or argon may be reduced in one or more of the following ways:
Reduce/minimise inerts such as nitrogen, methane and ethane in the ethylene feed gas.
Reduce/minimise inerts such as nitrogen and argon in the oxygen feed gas.
Reduce/minimise inerts such as dissolved nitrogen in the acetic acid feed.
Reduce/eliminate inert gas purges to instruments, for example by using ethylene or carbon dioxide purges.
Use carbon dioxide as purge gas.
Reduce/minimise inerts from catalyst transfer operations or use ethylene or carbon dioxide.
Reduce/eliminate leakage of nitrogen into the oxygen via sparger purge system.
Reduce/eliminate air leakage into the vacuum system of product recovery.
Reduce/minimise inerts in water used in the off-gas prescrubber prior to the carbon dioxide removal system (Benfield).
Design the oxygen injection system to reduce/minimise the use of inert gas.
The oxygen-containing gas may suitably be air or a gas richer or poorer in molecular oxygen than air. Suitably, the gas may be oxygen diluted with a suitable diluent, for example, nitrogen, argon or carbon dioxide. Preferably the gas is oxygen. The amount of oxygen in the combined feed to the reactor is determined by factors such as conversion and the flammability limits of the gases exiting the reactor. Preferably, the amount of oxygen is the combined feed to the reactor is in the range 4 to 15 mol %, suitably 4 to 12 mol%.
The acetic acid may be introduced into the reactor in liquid form. Optionally, some acetic acid may be introduced in the vapour form. The acetic acid need not be highly purified, that is it may be crude acetic acid. Suitably, the liquid acetic acid may be introduced into the fluid bed reactor by any suitable injection means, for example by a nozzle in which a gas is used to assist in the atomisation of the liquid or, liquid-only spray-type nozzles may be used. Additionally, recycled acetic acid may be introduced into the reactor. The re-cycled acetic acid optionally containing small amounts of water may be pre-mixed with the crude acetic acid or may be introduced into the reactor using a separate injection means. Suitably the amount of acetic acid in the combined feed to the reactor is in the range of 10 to 20 mol % and the amount of water is less than 6 wt%, preferably less than 4 wt%, more preferably less than 3 wt% of the total of acetic acid and water entering the reactor.
The process is carried out in a reactor and may suitably be operated at a temperature of from 100 to 400xc2x0 C., preferably 140 to 210xc2x0 C.
The process may be carried out in a fixed bed or fluid bed reactor.
The unreacted ethylene may be recovered from the gases withdrawn from the reactor by the steps of (i) separating condensable products from the gases by condensation and (ii) treating the gases from step (i) to recover ethylene, for example by membrane separation or chemical treatment. The condensable products may be separated from the withdrawn gases by condensation for example by using a heat exchanger or by quenching the gases by flowing counter to a flow of liquid acetic acid and/or condensable products which absorbs the vinyl acetate product.
Chemical treatment may comprise the steps of (axe2x80x2) contacting the gases from step (i) with acetic acid in a scrubber to remove residual vinyl acetate product; (bxe2x80x2) treating the product of step (axe2x80x2) with water in a scrubber to remove acetic acid and (cxe2x80x2) removing carbon dioxide from the ethylene in the product for step (bxe2x80x2) by contacting with potassium carbonate. A Benfield system may be used.